I know there is MemTest86+ for PCs, but what can I use to see if my RAM is actually good in an Apple product? Whether it's a MacBook, MacBook Pro, iMac or Mac Pro? What symptoms would show that RAM is bad?

I had an iMac G5 that booted for months with bad ram. The symptom I saw was infrequent (but more than I was used to) kernel panics. I finally associated the panics with my having installed a memory upgrade. (I can be slow). Confirmed bad module (using Rember), swapped with vendor, never saw another panic—or not so as I remember,
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jabergJan 25 '12 at 18:48

Kernel panics, the black screen & beeps (referenced elsewhere), or weird crashes that you can't attribute to anything else all can be caused by bad RAM.

I use Rember which provides a GUI interface to Memtest. It was suggested to me by a tech support rep at One World Computing as their approved way of checking for a bad module before returning ram for exchange. I didn't want to waste their time and resources if my problem, periodic kernel panics, wasn't, as I suspected, caused by a memory upgrade. It turned out that the memory was to blame.

Memtest and Rember will sometimes find problems that Apple's Hardware Test won't and I highly recommend using one of them if you're looking for bad memory modules.

Some signs of bad RAM also include black screen 3 beeps in 5 seconds, this may happen continuously, You can use the method that EpiGrad mentioned or alternatively you can test it with Apple's Hardware Test, if you still have the OSX Disc, insert disc, hold down d on start up memory test should be one of the main categories listed, unless it has been updated since i last used it.